The car: 1989 260E (103 engine) with over 300K on the clock
The problem: Backfire at start up (50% of the time) when it has reached normal operating temperature. If the car sits, say for 10 minutes, when I start it a loud (POW) backfire happens in the intake maniforld.
The cars has new plugs, wires, cap, rotor, ignition module, valve guides/seals, new gas filter, fuel pressure regulator, All parts replaced are OEM. Compression at 165 on all 6, burns no oil, excellent gas mileage, runs smooth, no idle problem, good power (for a 260E). Always start on the first try either cold , very cold (minus 35 last winter) or hot, no overheating. No gas smell. No play in the timing chain.
The problem was present before I had the ignition system was renewed.
When the backfire happens, the car will start with no problem afterwards.
Any idea? Thanks
jackd

When you changed the sparkplugs, did you notice one that looked quite different from the others ( carbon build-up )?
One way of checking for a dripping injector would be to observe a pressure gauge reading ( tapped into the fuel rail ).
The best method would probably be on a testbench.
Depending on how much $$$$ you want to spend, you could even have the injectors cleaned, flow-tested and ultimately "blueprinted".

When I replaced the sparkpugs, they had about 35K on them.
I and my mechanic were surprised at the condition they were. They all looked the same with a nice brownish/tan color, no deposit of any kind and barely any wear on the electrodes.
A less fussy owner would probably have reinstalled them.
jackd

Firing order is 1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4-1-5-3-6-2-4. It does that about 600 time per minute at idle.
Seriously, Are you talking about cross-fire from the wires?.
I also tought about that possibility when I had the old sparkplugs wires. (Same backfires then). Same thing with the new wires.
It only does it when the engine is at normal runnig temperature and has been sitting off for 5 to 15 minutes.
jackd

I checked with my trusty mechanic yesterday about the possibility of having some leaky injectors.
As I said, I had some head surgery (valve guides, seals, new camshaft etc) done a few months ago. At that time, all 6 injectors were replaced. The problem was there before doing the work and is still present after the new injectors were installed.
Does this exclude the injectors as being the culprit?
jackd

Not being familiar with your engine, do you have a fuelpressure regulator with a vacuum line running to the intake manifold.
Just wondering if it's getting a little bit of fuel from a suspect diaphragm ?